Spin Torque Transfer (STT) Magnetoresistive Random Access Memory (MRAM) is an attractive emerging memory technology, offering non-volatility, high performance, and high endurance. An STT MRAM memory cell generally includes a Magnetic Tunnel Junction (MTJ) in series with a Field Effect Transistor (FET) which is gated by a Word Line (WL). A Bit Line (BL) is connected to the MTJ and runs perpendicular to the WL. A Source Line (SL) is connected to the FET and may run parallel to, and at the same pitch as, the BL.
Alternatively, the SL may be common to all cells within a region of the array, in what is known as a Common Source Line (CSL) configuration. See, for example, Zhao et al., “Architecting a common-source-line array for bipolar non-volatile memory devices,” 2012 Design, Automation & Test in Europe Conference & Exhibition (DATE), pgs. 1451-1454 (March 2012). The CSL configuration is much preferred as it eliminates the need for SLs to run through the array parallel to, and at the same pitch as, the BLs. It also reduces the need for SL support circuits. However, the CSL configuration places limitations on the methods used to write the cell, as all cells within that region share the same SL.
In standby, the WL, BL, and SL are held to ground. One cell along the BL is selected by raising its WL. When a sufficiently large voltage is forced across the cell from BL to SL, the selected MTJ is written to a particular (i.e., parallel or anti-parallel) state. The written state is determined by the polarity of this voltage (BL high versus SL high).
When the cell is in the parallel or P state, the resistance (R) of the MTJ is lower than when in the anti-parallel or AP state. Representative values might be RP=10 KΩ and RAP=20KΩ. The selected cell is read by sensing the resistance from BL to SL. The sensing or read voltage needs to be much lower than the write voltage to avoid disturbing the cell. The state-dependent change in resistance is characterized by the parameter magnetoresistance or MR, which is defined as MR=(RAP−RP)/RP. For example, 100% is a typical nominal value for MR although higher values have been reported.
For read, the selected BL current is sensed while forcing the BL to Vread, perhaps 100 mV above the SL which is held to ground. The data cell current is typically compared to a reference current to determine the state of the cell. A common method of generating this reference current is through the use of reference cells, which have been pre-programmed to specific states and which are configured so as to produce a reference current roughly midway between the two nominal data state currents.
Conventional reference configurations, however, have some notable drawbacks in terms of symmetry between the data and reference sides. This mismatch prohibits the use of certain advantageous sensing techniques.
Therefore, improved STT MRAM reference cell configurations would be desirable.